Quantification of organ function by SPECT and PET scintigraphic imaging is deleteriously affected by patient movement, unavoidable inclusion of scattered radiation in the acquired images, radiation attenuation, and by a number of other physical factors. Removal or minimization of these effects would improve the ability of these methods to correctly portray organ isotope content and thus improve lesion detectability and evaluation of organ status. During this reporting period efforts were continued to develop a spatial tracking system for monitoring head movement during PET imaging and to use these position data acquired during the scan to correct the image data for this movement. The tracking system referred to in the previous reporting period was redesigned as a stereoscopic system to simplify system operation. This system was tested on the benchtop and tests have begun in volunteer subjects in the actual scanning environment to identify "human factors" problems both in setup and in patient tolerance. This system should be available for use in the coming year. In addition, efforts were undertaken to develop a method for reducing the scatter content of planar and SPECT IMAGES. With this method, energy spectra are acquired at every image point and scatter is removed from each spectra by curve-fitting each spectra with an a priori scatter function that describes the amount of scatter falling in the photopeak region of each spectra. This method was tested in phantoms of increasing complexity and found to markedly improve the quantitative accuracy of the imaging process.